Hydride 142100 221750251 2008-06-25T22:43:32Z 171.69.47.115 /* Transition metal hydrido complexes */ '''Hydride''' is the name given to the [[Electric charge|negative]] [[ion]] of [[hydrogen]], H<sup>−</sup>. Although this ion does not exist except in extraordinary conditions, the term hydride is widely applied to describe [[Chemical compound|compounds]] of hydrogen with other [[chemical element|elements]], particularly those of [[Periodic table group|groups]]&nbsp;1&ndash;16. The variety of compounds formed by hydrogen is vast, arguably greater than that of any other element. Various metal hydrides are currently being studied for use as a means of hydrogen storage in [[fuel cell]]-powered electric cars and [[Battery (electricity)|batteries]]. They also have important uses in [[organic chemistry]] as powerful [[reducing agent]]s, and many promising uses in [[hydrogen economy]]. Every element of the [[periodic table]] (except some [[noble gas]]es) forms one or more hydrides. These compounds may be classified into three main types by the predominant nature of their [[Chemical bond|bonding]]: *''Saline hydrides'', which have significant ionic character, *''Covalent hydrides'', which include the hydrocarbons and many other compounds, and *''Interstitial hydrides'', which may be described as having [[metallic bonding]]. == Hydride ion == :''See also:'' [[hydrogen anion]]. Aside from [[electride]], the hydride ion is the simplest possible [[anion]], consisting of two [[electron]]s and a [[proton]]. Hydrogen has a relatively low [[electron affinity]], 72.77&nbsp;kJ/mol, thus hydride is so basic that it is unknown in solution. This however is deceptive since the proton is so acidic it is also unknown in solution. The reactivity of the hypothetic hydride ion is dominated by its exothermic protonation to give [[dihydrogen]]: ::H<sup>−</sup> + H<sup>+</sup> → H<sub>2</sub>; [[Enthalpy|Δ''H'']] = −1676&nbsp;kJ/mol As a result, the hydride ion is one of the strongest [[base (chemistry)|base]]s known. It would extract protons from almost any hydrogen-containing species. The low electron affinity of hydrogen and the strength of the H&ndash;H bond (∆H<sub>BE</sub> = 436&nbsp;kJ/mol) means that the hydride ion would also be a strong [[reducing agent]]: ::H<sub>2</sub> + 2e<sup>−</sup> {{unicode|⇌}} 2H<sup>−</sup>; [[Standard electrode potential|''E''<sup><s>o</s></sup>]] = −2.25 V == Ionic hydrides == In ionic, or saline, hydrides, the hydrogen is viewed as a [[pseudohalide]]. The saline hydrides are insoluble in conventional solvents, reflecting their nonmolecular structures. H<sup>−</sup> has stable [[electron configuration]] of [[helium]] with a filled 1s-orbital. Ionic hydrides also feature an electropositive metal, usually one of the [[alkali metals]] or [[alkaline earth metals]]. These hydrides are called binary if they only involve two elements including hydrogen. [[Chemical formula]]e for binary ionic hydrides typically MH (as in [[lithium|Li]]H). As the charge on the metal increases, the M-H bonding becomes more covalent as in [[magnesium|Mg]]H<sub>2</sub> and [[aluminium|Al]]H<sub>3</sub>. Ionic hydrides are commonly encountered as basic [[reagent]]s in [[organic synthesis]]: :[[Acetophenone|C<sub>6</sub>H<sub>5</sub>C(O)CH<sub>3</sub>]] + [[Potassium hydride|KH]] &rarr; C<sub>6</sub>H<sub>5</sub>C(O)CH<sub>2</sub>K + H<sub>2</sub> Such reactions are heterogeneous because the KH does not dissolve. Typical solvents for such reactions are [[ether]]s. [[Water]] cannot serve as a medium for pure ionic hydrides or LAH because the hydride ion is a stronger [[Base (chemistry)|base]] than [[hydroxide]]. Hydrogen gas is liberated in a typical acid-base reaction. :NaH + H<sub>2</sub>[[Oxygen|O]] → H<sub>2</sub> (gas) + NaOH Δ''H'' = −83.6&nbsp;kJ/mol, [[Gibbs free energy|Δ''G'']] = −109.0&nbsp;kJ/mol Alkali metal hydrides react with metal halides. [[Lithium aluminium hydride]] (often abbreviated as LAH) arises from reactions with [[aluminium chloride]]. :4 [[Lithium hydride|LiH]] + AlCl<sub>3</sub> → LiAlH<sub>4</sub> + 3 LiCl ==Covalent hydrides== In covalent hydrides, hydrogen is [[covalent bond|covalently bonded]] to more [[electronegativity|electropositive]] element such as p-block ([[boron]], [[aluminium]], and Group 4-7) elements as well as [[beryllium hydride|beryllium]]. Common compounds include the [[hydrocarbons]] and [[ammonia]] could be considered as ''hydrides'' of [[carbon]] and [[nitrogen]], respectively. Charge neutral covalent hydrides that are molecular are often volatile at room temperature and [[atmospheric pressure]]. Some covalent hydrides are not volatile because they are polymeric—i.e. nonmolecular—such as the binary hydrides of aluminium and beryllium. Replacing some hydrogen atoms in such compounds with larger [[ligand]]s, one obtains molecular derivatives. For example, [[diisobutylaluminium hydride]] (DIBAL) consists of two aluminium centers bridged by hydride ligands. Hydrides that are soluble in common solvents are widely used in [[organic synthesis]]. Particularly common are [[sodium borohydride]] (NaBH<sub>4</sub>) and lithium aluminum hydride and hindered reagents such as DIBAL. ===Transition metal hydrido complexes=== {{main|Transition metal hydride}} Most transition [[metal complex]]es form molecular compounds that contain one or more hydride ligands. Usually such compounds are discussed in the context of [[organometallic chemistry]]. They are intermediates in many industrial processes that rely on metal catalysts, such as [[hydroformylation]], [[hydrogenation]], and [[hydrodesulfurization]]. Deprotonation of [[dihydrogen complex]]es gives metal hydrides. Two famous examples of transition metal hydrides are HCo(CO)<sub>4</sub> and H<sub>2</sub>Fe(CO)<sub>4</sub>, are acidic thus demonstrating that the term hydride is used very broadly. The anion [[Potassium nonahydridorhenate|[ReH<sub>9</sub>]<sup>2−</sup>]] is a rare example of a molecular [Homoleptic] metal hydride. ==Interstitial hydrides of the transitional metals== Structurally related to the saline hydrides, the transition metals form binary hydrides which are often non-[[stoichiometric]], with variable amounts of hydrogen atoms in the lattice, where they can migrate through it. In [[materials engineering]], the phenomenon of [[hydrogen embrittlement]] is a consequence of interstitial hydrides. [[Palladium]] absorbs up to 900 times its own volume of hydrogen at room temperatures, forming [[palladium hydride]], and was therefore once thought as a means to carry hydrogen for vehicular [[fuel cell]]s. Hydrogen gas is liberated proportional to the applied temperature and pressure but not to the chemical composition. Interstitial hydrides show certain promise as a way for safe [[hydrogen storage]]. During last 25 years many interstitial hydrides were developed that readily absorb and discharge hydrogen at room temperature and atmospheric pressure. They are usually based on [[intermetallic]] compounds and solid-solution alloys. However, their application is still limited, as they are capable of storing only about 2 weight percent of hydrogen, which is not enough for automotive applications. ==Nomenclature== The following is a list of the nomenclature for the hydride derivatives of main group compounds: *[[alkali metal|alkali]] and [[alkaline earth metal|alkaline earth]] metals: metal hydride *[[boron]]: [[borane]] and rest of the group as metal hydride *[[carbon]]: [[alkane]]s, [[alkene]]s, [[alkyne]]s, and all [[hydrocarbon]]s *[[silicon]]: [[silane]] *[[germanium]]: [[germane]] *[[tin]]: [[stannane]] *[[lead]]: [[plumbane]] *[[nitrogen]]: [[ammonia]] ('azane' when [[substituent|substituted]]), [[hydrazine]] *[[phosphorus]]: [[phosphine]] ('phosphane' when substituted) *[[arsenic]]: [[arsine]] ('arsane' when substituted) *[[antimony]]: [[stibine]] ('stibane' when substituted) *[[bismuth]]: [[bismuthine]] ('bismuthane' when substituted) According to the convention above, the following are "hydrogen compounds" and not "hydrides": *[[oxygen]]: [[water]] ('oxidane' when substituted), [[hydrogen peroxide]] *[[sulfur]]: [[hydrogen sulfide]] ('sulfane' when substituted) *[[selenium]]: [[hydrogen selenide]] ('selane' when substituted) *[[tellurium]]: [[hydrogen telluride]] ('tellane' when substituted) *[[halogen]]s: hydrogen halides Examples: *[[nickel hydride]]: used in [[NiMH battery|NiMH batteries]] *[[palladium hydride]]: electrodes in [[cold fusion]] experiments *[[lithium aluminium hydride]]: a powerful reducing agent used in organic chemistry *[[sodium borohydride]]: selective specialty reducing agent, hydrogen storage in [[direct borohydride fuel cell|fuel cells]] *[[sodium hydride]]: a powerful base used in organic chemistry *[[diborane]]: reducing agent, rocket fuel, semiconductor dopant, catalyst, used in organic synthesis; also [[borane]], [[pentaborane]] and [[decaborane]] *[[arsine]]: used for [[Doping (semiconductor)|doping]] [[semiconductors]] *[[stibine]]: used in [[semiconductor]] industry *[[phosphine]]: used for [[fumigation]] *[[silane]]: many industrial uses, e.g. manufacture of [[composite material]]s and water repellents *[[ammonia]]: [[coolant]], [[fertilizer]], many other industrial uses *[[hydrogen sulfide]]: component of [[natural gas]], important source of [[sulfur]] *Chemically, even [[water]] and [[hydrocarbon]]s could be considered hydrides. ===Isotopes of hydride=== ''Protide'', ''deuteride'', and ''tritide'' are used to describe ions or compounds, which contain [[isotopic enrichment|enriched]] [[hydrogen-1]], [[deuterium]] or [[tritium]], respectively. ===Precedence convention=== According to [[IUPAC inorganic nomenclature|IUPAC convention]], by precedence (stylized electronegativity), hydrogen falls between [[nitrogen group|group 15]] and [[chalcogen|group 16]] elements. Therefore we have NH<sub>3</sub>, 'nitrogen hydride' ([[ammonia]]), versus H<sub>2</sub>O, 'hydrogen oxide' ([[water]]). ==See also== *[[Hydron (chemistry)]] *[[Hydronium]] ==External links== * [http://www.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap6-3.04.pdf IUPAC hydride nomenclature] [[Category:Hydrides|*]] [[Category:Anions]] [[cs:Hydrid]] [[de:Hydride]] [[es:Hidruro]] [[fr:Hydrure]] [[it:Idruro]] [[he:הידריד]] [[lv:Hidrīdi]] [[nl:Hydride]] [[ja:水素化合物]] [[nds:Hydriden]] [[pl:Wodorki]] [[pt:Hidreto]] [[ru:Гидриды]] [[fi:Hydridi]] [[sv:Metallhydrid]] [[uk:Гідриди]] [[zh:氢化物]]